Wow, fantastic project! Using the shadows like this was a really cool idea, you had a great aesthetic for a puzzle game. The inspiration from Portal was very clear and endearing.
I think a very short tutorial or story element could add a lot to the game. It took me a while to figure out what I needed to do, and I didn't always understand a clear motivation or goal for each stage.
I liked the sparse, mysterious vibe, but I still think adding music to the levels could go a long way without getting rid of that.
Nicely done! I would definitely be interested in playing a fleshed-out version of this.